1. Field of the Invention
This invention relates to a process for preparing x-ray contrast compositions containing radiopaque nanoparticles. 2. Reported Developments
X-Ray imaging is a well known and extremely valuable tool for the early detection and diagnosis of various disease states in the human body. The use of contrast agents for image enhancement in medical x-ray imaging procedures is widespread. An excellent background on contrast agents and media in medical imaging is provided by D. P. Swanson et al., Pharmaceuticals in Medical Imaging, 1990, MacMillan Publishing Company, the disclosure of which is hereby incorporated by reference in its entirety.
Briefly, in x-ray imaging, transmitted radiation is used to produce a radiograph based upon overall tissue attenuation characteristics. X-rays pass through various tissues and are attenuated by scattering, i.e., reflection or refraction or energy absorption. However, certain body organs, vessels and anatomical sites exhibit so little absorption of x-ray radiation that radiographs of these body portions are difficult to obtain. To overcome this problem, radiologists routinely introduce an x-ray absorbing medium containing a contrast agent into such body organs, vessels and anatomical sites.
Currently available x-ray contrast agents generally exhibit a lack of site directed delivery or compartmentalization. Consequently, large quantities of agent are normally required for imaging. It would be desirable to restrict the contrast agent to specific biological or anatomical compartments, such as the blood pool, liver, kidney or spleen. This would reduce the overall amount of agent which needs to be administered to achieve the desired contrast enhancement.
Maximum enhancement of major blood vessels takes place during the so-called vascular phase of contrast media kinetics which occurs within about the first two minutes following the intravascular infusion or bolus injection of the contrast media. This is because the plasma concentration of an intravascular contrast medium decreases rapidly as a result of vascular mixing transcapillary diffusion of the medium from the circulation into the interstitial spaces and renal excretion. Consequently, imaging of blood vessels must take place within a narrow time window, typically within a few minutes after infusion or injection of the x-ray contrast agent. Currently, there is no commercially available x-ray contrast agent for imaging blood vessels which provides good contrast images of the vasculature for an extended period of time. Therefore, multiple injections are often required to visualize the vasculature adequately. Furthermore, arteriography, as currently practiced, typically requires percutaneous or surgical catheterization, fluoroscopic localization and multiple bolus arterial administrations to adequately visualize a given vascular region.
The need for improved visualization of the liver, kidney and spleen, particularly for early detection of metastases, has led to numerous attempts at developing a contrast medium for accumulation by the mononuclear phagocyte system (MPS). In Handbook of Experimental Pharmacology, Vol. 73, Radiocontrast Agents, Chapter 13, "Particulate Suspensions as Contrast Media", Violante and Fischer describe and analyze the problems and complexities involved in designing and formulating such a medium. Inasmuch as the MPS of the liver and spleen is known to trap particles by phagocytosis, contrast agents in particulate form, such as emulsions of iodinated oils, e.g., iodinated ethyl esters of poppy seed oil, and liposomes containing water-soluble iodinated contrast agents have been proposed for liver and spleen visualization. However, emulsions tend to be unacceptably toxic when administered both intravenously and subcutaneously and liposomes tend to require unacceptably large amounts of lipid to achieve adequate contrast enhancement. The MPS or Kuppfer cells of the liver, to which liposomes and emulsions have been directed, constitute approximately 5 percent of the total cell population, the remainder being hepatocyte cells.
Submicron inorganic radioactive thorium dioxide particles have been used for liver visualization and have shown effective contrast enhancement in clinical testing. However, their use has been discontinued because of the extremely lengthy retention of the particles in e liver. This, in combination with the inherent radioactivity of thorium, has led to serious adverse side effects including neoplasm and fibrosis.
Violante et al, U.S. Pat. No. 4,826,689, disclose a method of making uniformly sized noncrystalline amorphous particles from water-insoluble organic compounds wherein the organic compound is dissolved in an organic solvent. In one embodiment, iodipamide ethyl ester is dissolved in dimethylsulfoxide. However solvent precipitation techniques such as described in U.S. Pat. No. 4,826,689 for preparing particles tend to provide solvent contaminated particles. Such solvents are often toxic and can be very difficult, if not impossible, to adequately remove to pharmaceutically acceptable levels for diagnostic imaging. Additionally, amorphous materials and formulations tend to exhibit unacceptably poor stability and/or short shelf-lives.
Motoyama et al, U.S. Pat. No. 4,540,602 disclose that a solid drug can be pulverized in an aqueous solution of a water-soluble high molecular substance, and that as a result of such wet grinding, the drug is formed into finely divided particles ranging from 0.5 .mu.m or less to 5 .mu.m in diameter. However, there is no suggestion that particles having an average particle size of less than about 400 nm can be obtained. Indeed, attempts to reproduce the wet grinding procedures described by Motoyama et al resulted in particles having an average particle size of much greater than 1 .mu.m.
PCT/EP90/00053 describes water-insoluble iodinated carbonate esters reported to be useful as contrast agents for visualization of the liver and spleen. Particles of mean diameter on the order of 1.0 micron of the disclosed esters reportedly are taken up by the reticuloendothelial system of the liver and spleen. However, such particles are prepared by conventional mechanical crushing or spray drying techniques or by solvent precipitation techniques such as described in U.S. Pat. No. 4,826,689.
Recently, the prior art has reported production and utilization of nanoparticulate crystalline substances found to be desirable in both pharmaceutical compositions for prevention and treatment of diseases and radiopaque compositions for detection of abnormalities in soft tissues used in conjunction with radiographic examinations.
Methods of making finely divided drugs have been studied and efforts have been made to control the size and size range of drug particles in pharmaceutical compositions. For example, dry milling techniques have been used to reduce particle size and hence influence drug absorption. However, in conventional dry milling, as discussed by Lachman, et al., The Theory and Practice of Industrial Pharmacy, Chapter 2, "Milling", p.45 (1986), the limit of fineness is reached in the region of 100 microns (100,000 nm) when material cakes on the milling chamber. Lachman, et al. note that wet grinding is beneficial in further reducing particle size, but that flocculation restricts the lower particle size limit to approximately 10 microns (10,000 nm). However, there tends to be a bias in the pharmaceutical art against wet milling due to concerns associated with contamination. Commercial airjet milling techniques have provided particles ranging in average particle size from as low as about 1 to 50 .mu.m (1,000-50,000 nm).
Other techniques for preparing pharmaceutical compositions include loading drugs into liposomes or polymers, e.g., during emulsion polymerization. However, such techniques have problems and limitations. For example, a lipid soluble drug is often required in preparing suitable liposomes. Further, unacceptably large amounts of the liposome or polymer are often required to prepare unit drug doses. Further still, techniques for preparing such pharmaceutical compositions tend to be complex. A principal technical difficulty encountered with emulsion polymerization is the removal of contaminants, such as unreacted monomer or initiator, which can be toxic, at the end of the manufacturing process.
EPO 275,796 describes the production of colloidally dispersible systems comprising a substance in the form of spherical particles smaller than 500 nm. However, the method involves a precipitation effected by mixing a solution of the substance and a miscible non-solvent for the substance and results in the formation of non-crystalline nanoparticles. Furthermore, precipitation techniques for preparing particles tend to provide particles contaminated with solvents. Such solvents are often toxic and can be very difficult, if not impossible, to adequately remove to pharmaceutically acceptable levels to be practical.
U.S. Pat. No. 4,107,288 describes particles in the size range from 10 to 1,000 nm containing a biologically or pharmacodynamically active material. However, the particles comprise a crosslinked matrix of macromolecules having the active material supported on or incorporated into the matrix.
U.S. Pat. No. 5,145,684 discloses a process for preparing particles consisting of a crystalline drug substance having a surface modifier or surface active agent adsorbed on the surface of the particles in an amount sufficient to maintain an average particle size of less than about 400 manometers. The process of preparation comprises the steps of dispersing the drug substance in a liquid dispersion medium and applying mechanical means in the presence of grinding media to reduce the particle size of the drug substance to an average particle size of less than 400 nm. The particles can be reduced in the presence of a surface active agent or, alternatively, the particles can be contacted with a surface active agent after attrition. The presence of the surface active agent prevents flocculation/agglomeration of the nanoparticles.
The mechanical means applied to reduce the particle size of the drug substance is a dispersion mill, the variety of which include a ball mill, an attrition mill, a vibratory mill and media mill, such as sand mill, and a bead mill.
The grinding media for the particle size reduction is spherical or particulate in form and includes: ZrO.sub.2 stabilized with magnesia, zirconium silicate, glass, stainless steel, titania, alumina and ZrO.sub.2 stabilized with yttrium. Processing time of the sample can be several days long. This patent is incorporated herein in its entirety by reference.
To a more limited extent the prior art also utilized microfluidizers for preparing small particle-size materials in general. Microfluidizers are relatively new devices operating on the submerged jet principle. In operating a microfluidizer to obtain nanoparticulates, a premix flow is forced by a high pressure pump through a so-called interaction chamber consisting of a system of channels in a ceramic block which split the premix into two streams. Precisely controlled shear, turbulent and cavitational forces are generated within the interaction chamber during microfluidization. The two streams are recombined at high velocity to produce droplet shear. The so-obtained product can be recycled into the microfluidizer to obtain smaller and smaller particles.
The prior art has reported two distinct advantages of microfluidization over conventional milling processes (such as reported in U.S. Pat. No. 5,145,684, supra): substantial reduction of contamination of the final product, and the ease of production scaleup.
Numerous publications and patents were devoted to emulsions, liposomes and/or microencapsulated suspensions of various substances including drug substances produced by the use of microfluidizers. See, for example:
1) U.S. Pat. No. 5,342,609, directed to methods of preparing solid apatite particles used in magnetic resonance imaging, x-ray and ultrasound. PA1 2) U.S. Pat. No. 5,228,905, directed to producing an oil-in-water dispersion for coating a porous substrate, such as wood. PA1 3) U.S. Pat. No. 5,039,527 is drawn to a process of producing hexamethylmelamine containing parenteral emulsions. PA1 4) G. Gregoriadis, H. Da Silva, and A. T. Florence, "A Procedure for the Efficient Entrapment of Drugs in Dehydration-Rehydration Liposomes (DRVs)," Int. J. Pharm. 65, 235-242 (1990). PA1 5) E. Doegito, H. Fessi, M. Appel, F. Puisieux, J. Bolard, and J. P. Devissaguet, "New Techniques for Preparing Submicronic Emulsions--Application to Amphotericine-B,: STP Pharma Sciences 4, 155-162 (1994). PA1 6) D. M. Lidgate, R. C. Fu, N. E. Byars, L. C. Foster, and J. S. Fleitman, "Formulation of Vaccine Adjuvant Muramyldipeptides. Part 3. Processing Optimization, Characterization and Bioactivity of an Emulsion Vehicle," Pharm Res. 6, 748-752 (1989). PA1 7) H. Talsma, A. Y. Ozer, L. VanBloois, and D. J. Crommelin, "The Size Reduction of Liposomes with a High Pressure Homogenizer (Microfluidizer): Characterization of Prepared Dispersions and Comparison with Conventional Methods," Drug Dev. Ind. Pharm. 15, 197-207 (1989). PA1 8) D. M. Lidgate, T. Trattner, R. M. Shultz, and R. Maskiewicz, "Sterile Filtration of a Parenteral Emulsion," Pharm. Res. 9, 860-863 (1990). PA1 9) R. Bodmeier, and H. Chen, "Indomethacin Polymeric Nanosuspensions Prepared by Microfluidization," J. Contr. Rel. 12, 223-233 (1990). PA1 10) R. Bodmeier, H. Chen, P. Tyle, and P. Jarosz, "Spontaneous Formation of Drug-Containing Acrylic Nanoparticles," J. Microencap, 8, 161-170 (1991 ). PA1 11) F. Koosha, and R. H. Muller, "Nanoparticle Production by Microfluidization," Archiv DerPharmazie 321,680 (1988). PA1 a) dispersing an x-ray contrast substance in a liquid dispersion medium containing a surface modifier; and PA1 b) subjecting the liquid dispersion medium to the comminuting action of a microfluidizer asserting shear, impact and cavitation forces onto the x-ray contrast substance contained in the liquid dispersion medium for a time necessary to reduce the mean particle size of said x-ray contrast substance to less than 400 nm.
However, reports are few on reducing mean particle size (hereinafter sometimes abbreviated as MPS) of water-insoluble materials for use in pharmaceutical/diagnostic imaging compositions.
The present invention is directed to a process incorporating the advantages of microfluidizer process over conventional milling processes along with utilizing formulation and/or process parameters necessary for successful particle size reduction of a pharmaceutical suspension for x-ray contrast use prepared by microfluidization.
The primary forces attributed to microfluidization for producing either emulsions or dispersions, and for reducing the MPS of water-insoluble materials include: shear, involving boundary layers, turbulent flow, acceleration and change in flow direction; impact, involving collision of solid elements and collision of particles in the chamber of microfluidizer; and cavitation, involving an increased change in velocity with a decreased change in pressure and turbulent flow. An additional force can be attributed to conventional milling processes of attrition, i.e., grinding by friction. In reference to conventional milling process it is understood that the process involves the use of gravity, attrition and/or media mills, all containing a grinding media.